Very specific demands have to be met by high temperature applications, e.g., piston rings that are intended for use in for instance marine diesel engines, particularly as concerns strength, anti-corrosive properties, wear resistance, and material resilience. When used in a diesel engine the piston ring is arranged to abut on the one hand against an associated piston groove, on the other against an engine cylinder-bore.
Consequently, the ring should be wear-resistant, particularly at the interface towards the cylinder bore, where high friction is generated when the engine is in operation. The piston ring should therefore also possess an inherent tension or resilience whereby the piston ring will constantly be forced outwards, into abutment against the cylinder bore. In addition, upon each explosive stroke of the engine, the piston ring is urged with considerable force radially outwards, into abutment against the cylinder bore, with consequential increase of stress. Due to a high working temperature in engines and especially due to the impact of produced heat, from contact between piston rings and cylinder liner during the process, many materials loose some of their yield strength and show softening.
In operation, especially some contact areas between the piston ring and cylinder liner material are exposed to high temperatures, to considerable temperature differences, and to the effects of a highly corrosive environment.
In order to withstand the effects of these stress-inducing causes, the piston ring therefore also must exhibit considerable wear resistance, ductility, and thermal stability. By ductility is to be understood herein the maximum possible deformation of the material before cracking begins.
Today, piston rings are generally manufactured from a cast-iron blank, which meets the requirements imposed on the material as regards strength and resilience but not on wear resistance on the surface thereof that faces the cylinder bore. Cast iron does not possess the required thermal stability at high temperature. A cast-iron piston ring blank therefore usually is provided with a wear-resistant wear layer on the surfaces most exposed to wear.
The wear layer, which usually is formed by a chromium-compound material, is generally applied to the piston ring blank in an electrolysis process as described e.g. in EP 0 668 375. In accordance with the teachings of this specification the piston ring blank is given a hard chromium layer in an electrolysis process. However, difficulties do arise in achieving a sufficiently strong bond between the material of the blank and the material of the wear layer, which causes problems, because of the risk that the material of the wear layer be torn away from the material of the blank. When this happens, the comparatively soft material of the blank-material surface is exposed to wear in the area of contact against the cylinder bore, with resulting considerable shortening of the life of the piston ring.
Another problem is that the coating gradually wears away, even if the bond between the surfaces is comparatively strong. The wear on the piston ring progresses slowly as long as the wear layer is intact but very rapidly, once that layer has disappeared. As a result, it may be difficult to determine in time when a piston ring change should be made.
Another issue is to increase the oxidation resistance of the piston ring at high temperature. In WO9532314 there is provided a nickel-aluminum intermetallic basis alloy considered suitable for pieces, such as gas turbine blades, exposed to a high and continuous thermal stress. This alloy is claimed to improve thermal, oxidation and thermo-shock resistance. However, the hardness properties and wear-resistance of this compound are considered insufficient for use by other pieces and elements exposed to wear and thermal stress.